MF/HF

MF/HF ( Medium Frequency and  High Frequency)
This definition specifically refers to the frequency spectrum but communic\tions equipment and broadcast receivers operating within this spectrum are referred to as MF/HF equipments.

The MF/HF spectrum is defined as the part of the electromagnetic spectrum which includes any radiation with a wavelength between 100 and 10 metres and a frequency between 3 and 30 megahertz.

Both MF and HF electromagnetic waves have their own particular characteristics and the frequencies that should be used for transmission vary depending on the distance of the receiving station from the transmitter and the time of day.

MF
MF communications depend mainly on ground-wave propagation but with a further reduction in range because of the increased effect of attenuation by the earth. However, sky-wave propagation starts to become significant at MF, particularly at night, greatly extending the range. This can be a negative effect, however, owing to mutual interference between stations on the same frequency, and interference fading caused by signals arriving at the receiver by different paths from the transmitting station.

A coast station can achieve good ground-wave coverage for voice communications up to 300 nautical miles. Ship stations, with less powerful transmitters and less elaborate antenna systems, can usually expect reliable ground-wave commun-ications up to 150 nautical miles for voice communications and 300 nautical miles for DSC/telex.

HF
In practice, a good guide to establishing reliable communication at HF is to monitor the telex (NBDP) channels of the wanted coast station on the more likely bands for the time of day and season and then to call the station on whichever band provides a strong stable signal. If this in not successful, the other bands should be tried. The ionosphere can behave erratically at times and, on occasion, reception is better in the ship-to-shore direction than in the shore-to-ship direction or vice versa. Communication is frequently unreliable around sunrise and sunset.

The considerable variability of radiocommunication at HF is a consequence of signal propagation being predominately by sky wave, both day and night. A ground-wave signal is still present but attenuates too rapidly to be of value for reliable commercial communications.

The D-layer of the ionosphere has little effect above 4 MHz and long-distance propagation is by reflection from the E- or F-layers. In general terms, the higher the HF band used, the greater the range. This is because the higher the frequency, the further the wave has to pass into the ionosphere before it undergoes sufficient bending to be returned to earth. To a first approx-imation, therefore, the situation is that the higher the frequency, the greater will be the reflection (mirror) height and so the greater will be the potential range.

Long-range propagation is also possible as a result of multiple reflections between the ground, the ionosphere and even between the layers of the ionosphere itself. However, these modes of transmission are very variable and would not be used intentionally for normal commercial communications.

The best policy for reliable HF communications is to use the highest frequency consistent with the length of the radio circuit using a single reflection. The angle at which a radio wave enters the ionosphere is also an important factor, with reflection occurring at a lower height for oblique incidence compared to vertical incidence.

The highest frequency which can be used to communicate between two fixed points by sky-wave propagation is known as the maximum usable frequency, MUF. Since this frequency puts the receiver on the edge of the skip distance, it is better to use the lower frequency of 0.85 x MUF, termed the optimum traffic frequency, in order to improve reliability.

Note, however, that the preferred choice of channel may already be in use.
For example, to establish communications with Lyngby (Denmark) during the daytime, the following would normally apply:

4 MHz  = N. France
6 MHz  = N. Spain
8 MHz  = N. Africa
12 MHz = Ghana
16 MHz = Angola
22/25 MHz = South Africa

At night, due to changes in the ionosphere, the situation changes as the F1 and F2 layers merge and the heights of the E and F layers fall. The general result is that, to cover the same range at night it is necessary to halve the operating frequency; e.g., a link from Portishead to Capetown during daytime is possible on 22/25 MHz, but during the night the 12 MHz bands would be the first choice.

When transmitting east�west, the signal may pass from daytime to night-time conditions, and it may be very difficult to establish effective communications. One strategy is to estimate the opti-mum transmission band according to the day/night conditions at the mid-point of the radio circuit. The best course of action may be to wait until the entire path between the two stations is in daylight or darkness.

Call setup for MF/HF is now normally accomplished using Digital Selective Calling procedures.